I'm trying to work out if this is a massive swerve move, or whether Warner Bros. have really let the casting cat out of the bag.
With Ryan Reynolds (last seen as Deadpool, easily the best part of
Wolverine) having been cast as the Green Lantern himself, the search is now on for the rest of the cast - namely, the love interest, main villain, sub-villain and guardian guide. Let's start by taking a more detailed look at the Lantern.
HAL JORDAN/ GREEN LANTERN

Hal Jordan, a test pilot, finds a crashed spaceship with a dying alien inside. The alien, Abin Sur, is hunting for an individual who is utterly honest and born without fear and -what luck! - Jordan fits the bill and receives the special ring that allows him to become the Green Lantern, one of the Green Lantern Corps, which are essentially space police.

His powers - checks Wikipedia - are as follows:
-Constructs of green 'solid-energy,' often of tremendous size and/or complexity.
-Flight, including flight at speeds beyond that of light.
-Instantaneous flight across the galaxy and to other galaxies through wormholes.
-Plasma bolts.
-Ability to walk through walls by traveling through 'the Fourth Dimension'
-The rings can act as semi-sentient computers, including accessing the Book of Oa, a massive database of everything from the laws of the Guardians and the Corps to the history of the universe.
-Time travel (One solitary power ring cannot generate enough power for time travel. Several power rings are needed to complete this feat.)
-The rings are still reliant on the lantern-shaped power batteries, but no longer limited to 24 hours' charge as they originally were. Kyle Rayner's ring was the first ring to absorb more power than originally thought, having stored the main power battery's energy following its explosion on Oa.
-Telepathic powers.
-Translation of virtually all languages.
-Force field generation.
-Radiation, including simulated kryptonite radiations.
-Generate "earplugs" to block out all telepathic communication and manipulation.
-Render user invisible.
-Green Lantern: Rebirth revealed that only a certain type of willpower can use the ring effectively, as evidenced when Green Arrow's "cynical" willpower barely allows him to generate a single arrow and leaves him exhausted after this feat.
-Put humans into a state of suspended animation.
-Fast healing of wounds.
-Size manipulation allowing ringbearer to grow or shrink in size.
-In Green Lantern: First Flight Sinestro was able to "reconnect synapses" in the brain of a dead criminal in order to extract information via a kind of guided discussion.
Apparently the only weaknesses he has are a peculiar aversion to the colour yellow and the fact that his ring won't work if he feels fear. Not sure how someboy this powerful will ever be able to feel fear, but, still.
Now for the other characters:
CAROL FERRIS

"26-32, a smart and attractive woman with an MBA. She started as a pilot and now runs Ferris Aircraft. She is a work-a-holic...FEMALE LEAD"
Carol Ferris began comic book life as a standard love interest, although, interestingly, she is Hal's boss. Later on in the series, she managed to get herself a power gem and became supervillain Star Sapphire.

Don't be surprised if Sapphire doesn't appear in this movie. She's a bit too big to be thrown away in the introduction - expect sequels.
DR. HECTOR HAMMOND

"27-35, Character actor who is austere and impersonal, he is real "scummy." He is a pathologist and also the son of a Senator."
Hammond plays two roles here - on the one hand, he's a potential love rival for Hal and Carol, on the other, he's also a supervillain. After an encounter with a fallen meteorite (think
Spider-Man 3's Venom for this), he takes it home and exposes the radiation on four kidnapped scientists. The radiation makes them incredibly smart but saps their wills, allowing him to force them to build incredible inventions that he sells on for a profit, becoming a massive celebrity. Later on in the comics, he exposes himself to the radiation and it mutates his brain, thus giving him superpowers and a head that no shower will be able to clean every part of.

SINESTRO

"Mid-30s to Mid-40s, he is smart, tough, intimidating and in great physical shape."
With a name like that, what do you expect? Naturally, he's the villain of the piece - that or a stage magician. Sinestro was one of the Green Lantern Corps who was expelled for using the powers for evil - you know the sort of stuff, stealing other people's pizzas, making people fear the Lantern symbol and being a dick in general. He was banished to another universe where he found the Weaponers, a group of people who hated the Guardians (the Lanterns) as much as he did. They made him the yellow ring (remember, the one weakness of the Lanterns) and he started waging a war against them, eventually creating the Sinestro Corps to battle the Lanterns. Expect that massively expensie CGI war to be featured in the sequels.
ABIN SUR

"Mid-30s to mid-40s, he is a great warrior."
Basically, he's Obi-Wan Kenobi. He'll die a short time into the film, probably at the hands of Sinestro, and Jordan will have to conquer his fear to be able to defeat him.
And for those of you wondering exactly how the movie will end, smartest guess will be the image of the Green Lantern taking to the sky as he says his famous oath:
"In brightest day, in blackest night,
No evil shall escape my sight
Let those who worship evil's might,
Beware my power... Green Lantern's light!"
Source:
http://spoilertv-movies.blogspot.com/2009/09/green-lantern-official-casting-call-and.html